The present invention relates to a rubber composition for coating a steel cord and a tire having a belt using the rubber composition.
Generally, since a heavy load is applied to tires for an automobile, a steel cord is used as a reinforcing material. Particularly during running, if a rubber and a steel cord are peeled off due to heat generation of tires, crucial breakdown is caused on the tires.
Conventionally, carbon black is mainly used as a reinforcing filler for a rubber composition for coating a steel cord. However, in recent years, there are increasing interests in environmental preservation on the earth and automobiles are not exceptional, thus, regulations for suppressing CO2 emission are reinforced. Further, since petroleum resources are limited and supply thereof has been decreasing year by year, oil prices are expected to increase in the future, and consequently, there is limitation on uses of materials derived from petroleum resources such as carbon black. In view of such a background, JP-A-2003-64222 discloses a means, wherein silica is used in tire parts. However, when silica is used, there is a problem that a Mooney viscosity of a rubber composition is increased, and processability of the rubber composition is deteriorated. In order to solve these problems, a means for lowering rolling resistance and improving processability is known, wherein specific silica is compounded to a rubber composition for coating a steel cord. When such specific silica which dose not lower processability is used, there is a problem that sufficient reinforcing property cannot be obtained.
Accordingly, in order to enhance reinforcing property, a method of reinforcing a rubber by a reaction of a silanol group of a silica surface with an ethoxy group of a coupling agent by using silica and a coupling agent is known. In this case, in order to accelerate this reaction during kneading, it is necessary to increase a kneading temperature to 140° C. or more. However, only by this, the reaction of a silanol group of a silica surface with an ethoxy group of a coupling agent does not proceed during kneading of a base rubber, and this reaction proceeds during a topping step where a steel cord is coated with a rubber composition, resulting in accumulation of generated ethanol in the coated rubber composition and thus a so-called blister phenomenon is generated. If this phenomenon occurs, there arise problems that necessity of decreasing a line speed during the topping step occurs, and a rubber thickness of a blistering portion becomes non-uniform, thus deteriorating adhesion between the steel cord of that portion and the rubber. In addition, an unreacted silane coupling agent reacts with a metal oxide layer of a roll, thereby causing a phenomenon that the rubber is not peeled off from the roll, which results in a problem that productivity is lowered.